I'm not exactly sure. Both my sisters draw pictures for me, and they both tend to have reoccurring characters with little "stories"--a bit of dialogue, or a tense scene of action, or just the aftermath of some crazy shenanigans. These two specifically have shown up twice or so in my personal gallery of family gift art, and each time my sister has changed things a little bit. (The eyepatch dude is actually supposed to have cat ears, but then I think she changed him to be more catlike in overall features.) The penguin has been silent throughout, but he has this air of menace in a way--maybe it's the rumpled trenchcoat and my knowldge of my sister's love of the character Rorschach from Watchmen that makes me get that vibe, though.
I've tried to encourage both my sisters to write a few short stories about their characters, but so far they resist my efforts. I thought giving a bit of fan art every now and then might at least encourage them to continue with the random-connected scenes they draw. But it seems like every time I do, their creative wells run into a dry spell. (Until they create something new, or gain inspiration for the old).